The quantity of fillers in LWC base paper is normally 5-15%. Typically, approximately half of the filler is derived from coated rejected material and half comprises fresh feed. Fillers are used to improve the optical and printing properties of the paper, although paper strength properties suffer simultaneously. This may result in runnability problems at the paper machine. Furthermore, filler pigments are less expensive than fiber. It is therefore profitable to keep the quantity of fillers as high as possible.
A process for making multilayer webs by multilayer web technology is known from FI Patent No. 92729. In the process, there is a paper machine comprising a multilayer headbox, and different pulps are formed in at least two separate distribution funnels from the same fresh pulp and from the same pulp material deposit. The fresh pulp is fed from the material deposit and divided into several suspension flows, which are fed to the multilayer headbox. “The chemicals and/or additives relevant to quality and making economy” of different paper qualities are fed into the suspension flows.
Multilayer web technology is also described in FI Patent No. 105 118 and in EP Published Patent Applications Nos. 824 157 and 1 152 086.
It has been observed that layering of the pulp improves paper surface smoothness. For example, if mechanical pulp containing high quantities of fines is layered on the surface layer, the smoothness of the layer structure will be improved. If, at the same time, bulky chemical or mechanical pulps are placed in the middle layer, the bulk of the paper can also be improved.
It has further been observed that layering of the filler onto the surface layers improves sheet smoothness compared with the surface of the non-layered sheet.
In known technology, multilayer web technology is recommended for use in objects, where the quality of paper or board has generally been improved by pre-coating. These are writing and printing papers, fine papers (coated and non-coated), LWC base papers and SC papers, for example.
Although several problems related to conventional technology have been removed, the surface layer of a multilayer product still requires good formation, which is manifested as good printability in addition to smoothness. Thus, poor formation can most clearly be seen in the grey shades of offset printing and as a mottled printing surface. In order to improve the retention of the ordinary fillers, they are used together with retention agents. These agents cause flocking, which further decreases formation.